The present invention relates to a method for heat-treating indium-doped dislocation-free gallium arsenide monocrystal with a view to improve properties of the gallium arsenide monocrystals.
Gallium arsenide monocrystal is grown horizontally by the horizontal Bridgman's method or vertically by the liquid encapsulated Czochralski (LEC) method. The monocrystal body obtained by means of the latter method is generally cylindrical except at the end portions (FIG. 4) so that wherever the body is cut by a plane normal to the axis of the body the cross section of the body is circular. This is propitious for obtaining circular wafers, which are the industrial standard wafers and are, therefore, acceptable as they are to the device processing lines where the wafers are further processed. Also the liquid encapsulated Czochralski method is regarded very highly for the facts that it is easier to render a wafer dislocation-free in this method and that it is relatively easy to obtain monocrystal gallium arsenide wafers of relatively large diameters in this method.
The gallium arsenide monocrystal is extensively used as a material to make substrates for various devices such as light emitting diode, light sensing device (i.e., light detector), devices for microwave communication system. Since the electron mobility is far greater in gallium arsenide monocrystal than it is in silicon semiconductor, the development of the substrates for integrated circuits having high electron mobility has become very active recently. Therefore, to reduce the density of or to completely eliminate the existence of the dislocation in gallium arsenide monocrystal is an increasingly acute subject of many researchers in this field of technology.
Incidentally, it is generally accepted that the stresses exerted on a crystal grown from the melt by a thermal shock is a chief cause of dislocations in the crystals.
As a result of the efforts made with the view of obtaining dislocation-free gallium arsenide monocrystal, various techniques have been developed to render the gallium arsenide monocrystal dislocation-free, and among them are procedures specially applicable in the liquid encapsulated Czochralski method. An example of the procedures usefully adopted in the liquid encapsulated Czochralski method is disclosed by B. C. Grabmaier et al., in Journal of Crystal Growth 13/14 635-639 (1972). According to the disclosure, the increase in dislocation density caused by sealing or a seal crystal itself in the grown crystal especially in the vicinity of the seed crystal/grown crystal interface is hindered by means of necking, which is a procedure comprising growing a 10 to 20 mm-long thin monocrystal portion having a diameter of 1 or 2 mm from the seed, and adopting a small temperature gradient at the seed crystal/grown crystal interface.
As a method for decreasing the dislocation density in gallium arsenide monocrystal, a technique is proposed wherein additives such as Si, Te, Sb, Al, In, and B are added to the gallium arsenide monocrystal. According to the method, these additives have a tendency of adhering to the dislocations as the dislocations develop in the growing crystal whereby the propagation and growth of the dislocations are prevented. Among the additives, indium has proved a successful dopant for industrially obtaining a dislocation-free gallium arsenide monocrystal. According to an example disclosed in Japanese Kokai No. 61-222991, it is possible to obtain dislocation-free gallium arsenide monocrystal without applying necking operation, if a molten gallium arsenide containing indium in an amount of about 6 weight % is prepared in a quartz crucible to be employed as the melt, and the seed to raise the monocrystal from the melt is doped with indium in an amount of 0.7 weight %. More generally, Japanese Kokai No. 61-222991 teaches that to obtain dislocation-free gallium arsenide monocrystal the seed should be doped with indium as much as kC weight % where k is a segregation coefficient of indium and C is the concentration of indium in the melt of gallium arsenide.
The indium-doped dislocation-free gallium arsenide monocrystal thus obtained is certainly free of dislocations, and from this fact it appears that the gallium arsenide is crystallographically uniform. However, it has been observed that thus grown indium-doped dislocation-free gallium arsenide monocrystal rod, as it is, exhibits ununiformity in axial direction in such respects as electric properties, the concentrations of traps such as EL2, the concentration of carbon that substituted arsenic, and occurrence of the micro precipitates. Even if such a monocrystal is in semi-insulation state, the monocrystal substrates cut therefrom will be such that when they are made into transistors by means of the ion-implantation procedure, the resulting transistors have inconsistent threshold voltages Vth, which determine the on-off positions in switching action.
An In-doped dislocation-free gallium arsenide monocrystal is ordinarily obtained by the liquid encapsulated Czochralski (LEC) method. As shown in FIG. 3, the liquid encapsulated Czochralski method employs a pressurized chamber (furnace) schematically drawn by the frame designated by 1, and this pressurized chamber 1 is filled with an inert gas. A PBN (pyrolytic boron nitride) crucible 2 is installed in the middle of the chamber 1 such that the crucible 2 is capable of being rotated around its center line by means of a shaft. Reference numeral 3 designates an In-doped melt of gallium arsenide contained in the crucible 2. The surface of the melt 3 is entirely covered with a layer of another melt 4, which is of B.sub.2 O.sub.3 whereby the melt 3 is sealed from the gaseous environment. A heating means 5 is provided to controllably heat the melt 3. The seed is lowered to the melt 4 and, penetrating the layer of the melt 4, it is caused to touch the surface of the melt 3. Then, while the crucible is slowly rotated by the shaft in the direction indicated by a curved arrow below (FIG. 3), the seed is moved upward very slowly as it is rotated in the opposite direction as indicated by a curved arrow above. The monocrystal grows from the lower tip of the seed as the seed is raised, and a monocrystal ingot 10 as shown in FIG. 4 is obtained in the end. In FIG. 3, reference numeral 6 designates a heat shield.
The In-doped dislocation-free monocrystal ingot or rod 10 thus obtained tends to have inconsistent properties along the axial direction, as stated above, and it is postulated that this is a result of the fact that the thermal history experienced by different portions of the monocrystal rod is not always the same. Especially the conditions under which the cooling process is conducted are difficult to maintain fixed or uniform in time with respect to the different portions of the monocrystal rod.
Koji Yamada et al reported in their preparatory treatise for the 47th Autumn Season Applied Physics Symposium, 22a-K-8 (1986), that as they analyzed an In-doped gallium arsenide monocrystal rod grown by the LEC method by means of the infrared transmission method (IRT) as well as the KOH etching method, they observed the existence of microdefects which were not dislocation in the monocrystal rod. Further, they reported in their preparatory treatise for the 34th Spring Season Applied Physics Symposium, 30a-Z-3 (1987), that the formation of the same microdefects was strongly dependent upon the thermal history below 1000.degree. C.
These reports showed that there exist microdefects which are not dislocations and are distributed ununiformly, in the In-doped dislocation-free gallium arsenide monocrystal rod and that the formation of the microdefects is affected by the thermal history during the monocrystal pulling-up operation. However, there has been disclosed no solution for making the distribution of the microdefects uniform throughout the monocrystal rod nor for eliminating the microdefects.
It is an object of the invention, therefore, to provide a method for thermally treating an indium-doped dislocation-free gallium arsenide monocrystal rod which method is capable of making homogeneous in the axial direction of the monocrystal rod the electric properties such as resistivity, the concentrations, i.e., the densities of deep traps such as EL2, the concentration of carbon that substituted arsenic at arsenic sublattice site (C.sub.As), and the density and size of microprecipitates, and thereby producing a gallium arsenide monocrystal from which it is possible to obtain high quality gallium arsenide monocrystal substrates desirous for manufacturing semiconductor devices.